Contrasting response of haematological variables between long-term training and short exercise bouts in zebra finches (Taeniopygia guttata)

Physical aerobic activity is oxygen-demanding, but – particularly for birds – there is still little understanding of how blood contributes to oxygen supply under various activity levels. In a two-factorial experimental design we investigated the long-term effect of daily flight training and the immediate effect of a short exercise bout on a set of haematological variables: haemoglobin content (HGB), haematocrit (HCT), red blood cell number (RBCcount) and size (RBCarea) in zebra finches (Taeniopygia guttata). For a period of six weeks, birds were either trained daily for 3 hours in a flight arena or remained untrained. Subsequently, half of each group was blood sampled either at resting condition or after a five-minute exercise bout in a flight-hover wheel. We found significantly lower HGB, HCT and RBCcount compared to untrained controls in response to training, while RBCarea did not differ between treatments. Response to an exercise bout revealed an opposite pattern with significantly higher HGB and HCT compared to non-exercised birds. Additionally, RBCarea was significantly smaller immediately after exercise compared to non-exercised birds, and such short-term flexibility represents a novel finding for birds. This contrasting response in erythrocyte characteristics with respect to long-term training and short exercise bouts appears as a clear pattern, presumably underlied by changes in water balance. We infer alterations of blood flow to be involved in adequate oxygen supply. During an exercise bout RBCarea flexibility may enhance not only oxygen delivery through improved erythrocyte surface-area-to-volume ratio, but also improve blood flow through a compensatory effect on blood viscosity.